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I've tried a bunch of broadband speed tests. Up until the last year or so, they usually agreed fairly closely when measuring my connection. Recently, the reported speeds have been much more diverse. I've read that Flash-based tests are all currently unreliable due to technical limitations in the current versions of Flash, so I've been trying non-Flash tests as well, but the results are inconclusive.

Why do the results vary so much? Is my ISP gaming the results of speed tests somehow? Is there a truly accurate test out there? I'd like to be able to say to my ISP that "test X is known to be accurate and it shows my speed is less than it should be."

Here are some of the results I've gotten recently, on a Shaw Cable plan in Canada (25 Mbps down; 2.5 Mbps up) (all speeds in Mbps - Down / Up):

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closed as not constructive by Ƭᴇcʜιᴇ007, Tog, Brad Patton, Dave M, Nifle Apr 21 '13 at 17:37

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Ask your ISP which one THEY trust. –  Ƭᴇcʜιᴇ007 Apr 20 '13 at 14:26
    
@techie007: They're going to tell me to trust their own test, listed above as "Shaw's speed test". But with my cynic hat on, it seems obvious that it's in their best interest to fudge those results. Unless you're saying that I should ask which one they trust, then NOT trust that one at all. –  boot13 Apr 20 '13 at 14:38
    
I'm suggesting that if you're going to complain about the speeds you're getting, you have to base it on the tool they trust, or they'll just blame the tool. :) Really as others have mentioned, time of day, weather, server traffic, network traffic, etc. ALL play a huge part in how accurate a speedtest is at any given time. –  Ƭᴇcʜιᴇ007 Apr 20 '13 at 14:41
    
@techie007: Although I didn't mention it in the question, the speeds I quoted are fairly consistent. The tests that show download speed at ~4 Mbps do that consistently; same for the ones that show download speed at ~24 Mbps. I've run the tests on weekdays, in the middle of the night, etc. and while there is some variation, the general result is basically the same. –  boot13 Apr 20 '13 at 14:54

2 Answers 2

It depends on many things. The current number of users your ISP is catering to (which usually leads to a slow down during peak hours). But the most appropriate reason, IMO, is the number of users the server which measures speed is catering to.

Suppose a server which is measuring speed has a total bandwidth of 100 MBps; and there are 5 users, say each having 40 MBps as their real speed. Then all 5 users who will test their speed simultaneously on this server (of 100MBps bandwidth) will get a result less than their real speed of 40 MBps.

Also, speeds somewhat depend on your distance to server. Hence speedtest usually always automatically chooses the server closest to you.

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Measuring the current throughput of a connection is trivial. I suspect none of the test would do this inaccurately. However, the maximum throughput as a limit imposed by your ISP is just one of the many factors affecting the actual speeds. Major other ones are server-side caps and the how much load your neighbours are putting on the network. ISPs generally don't reserve enough bandwith for each consumer level subscription in one area to be maxed out simultaneously.

There are dozens of smaller factors. A variable connection overhead, intentional slow-downs caused by the network protocol or quality of service settings, overloaded routers or infrastructure, low signal-to-noise ratios causing limited bandwith or packet loss, WLAN interference...

Speed tests do their best to minimise the impact of many of these factors, but you're not testing in a controlled environment. It's a bit like measuring your car's top speed on a busy road on your way to work. If the provider is not trustworthy, there's no way to tell for sure, but the best results from the list you provided appear to be fairly close to the advertised speeds, suggesting your ISP isn't cheating.

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